The present invention relates to a method for detecting a focussing error and a tracking error of an objective lens with respect to an object on which a light spot has to be focussed by said objective lens and to an apparatus for carrying out such a detecting method.
Such detecting method and apparatus are advantageously used in an apparatus in which a scanning light spot is projected by an objective lens onto one or more information tracks recorded spirally or concentrically on a disc-shaped record medium to read information recorded along the track.
In an embodiment of the apparatus for reproducing or picking-up information from the above mentioned record medium, the record medium is known as a video disc in which encoded video and audio signals are recorded as optical information such as optical transmittivity, reflection and phase properties. While the video disc is rotated at a high speed, such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source such as a helium-neon gas laser is focussed on the track of the disc as a light spot and the optical information is read out therefrom. One of the important properties of such a record medium is having a very high density of recorded information and thus a width of the information track that is very narrow and a narrow space between successive tracks of recorded information. In a typical video disc, the pitch of the tracks amounts only to 2 .mu.m. Therefore, the diameter of the light spot should be correspondingly small. In order to pick-up correctly the recorded information from tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. focussing error should be reduced as little as possible to make a spot diameter as small as possible.
To this end, the invention relates to apparatus which provides a focussing control system in which the amount and the direction of a de-focussed condition of the objective lens with respect to the disc surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of the objective lens in accordance with the detected focussing error signal.
Furthermore, during the reproduction, the light spot should follow the track precisely. For this purpose, the reproducing apparatus is also provided with a tracking control system in which an error in a position of the light spot with respect to the track, i.e. a tracking error is detected to produce a tracking error signal and the light spot is moved in a direction perpendicular to the track, i.e. a radial direction of the disc in accordance with the detected tracking error signal.
FIG. 1 is a schematic view illustrating a known focussing and tracking error detection system used in an optical pick-up apparatus. In this system, a tracking error and a focussing error are detected with the aid of a combination of a three-beam method and an astigmatism method. For this purpose, a laser light beam emitted from a laser light source 1 is divided by a diffraction grating 2 into three beams. These beams are transmitted by means of a lens 3, a beam splitting polarizing prism 4, a reflection surface 5 and a quarter-wavelength plate 6 into an objective lens 7. The three beams are focussed by the objective lens 7 as three light spots onto a disc 8 having one or more information tracks of crenellated pit construction.
FIG. 2 is a plan view showing a mutual relation between the three spots 10A, 10B and 10C and the track 9 formed in the disc 8. The spot 10A is situated at a center of the track 9 and the spots 10B and 10C are positioned at opposite side edges of the track 9. The center spot 10A is used to pick-up the information signal and to detect the focussing error signal. The spots 10B and 10C are used to detect the tracking error signal. The light beams reflected by the disc 8 are collected by objective lens 7 and made incident upon a light detector 12 by means of a quarter-wavelength plate 6, a mirror 5, a polarizing prism 4 and a cylindrical lens 11. Since the cylindrical lens 11 has a refraction power only in direction, the shape of the spot of the light beam corresponding to the center beam spot 10A is deformed with respect to the shape in an in-focussed condition in orthogonal directions, when the disc 8 moves up and down. As illustrated in FIG. 3, the light detector 12 comprises six light receiving regions 12A to 12F and four central regions 12A to 12D for receiving a central beam spot 10A' and two extreme regions for receiving two light beam spots 10B' and 10C', respectively, each beam spot 10B' and 10C' corresponding to the light spots 10B and 10C, respectively. Then, by suitably processing output signals from the regions 12A to 12D, it is possible to obtain the focussing error signal as well as the information signal. Further, the tracking error signal can be derived by producing a difference between output signals from the regions 12E and 12F.
In the known detection system so far explained with reference to FIGS. 1 to 3, three beams are produced by dividing the single laser beam emitted from the laser light source 1 by means of a grating 2. The light intensity of the respective beams is made small and thus, the gains of the detected signal are rather low. Further, since a relatively long optical path is required to focus the light beam after being reflected by the polarizing prism 4, there is a drawback that an optical system is liable to be large in size. Moreover, since the light detector 12 having the six regions must be arranged precisely in three axial directions, i.e. in the optical axis direction and in two orthogonal directions perpendicular to the optical axis, the adjustment in positioning the light detector is quite critical and requires time-consuming work. Furthermore, since a dynamic range in which the accurate focussing error signal can be obtained due to the deformation of the focussed beam spot 10A' is relatively small, any focussing error signal could not be produced if the disc 8 deviates from a given position even if only by a relatively small distance.
FIG. 4 is a schematic view showing another embodiment of the known focussing error and tracking error detection system disclosed in Japanese Patent Application Laid-open Publication No. 93,222/77. In this system, use is made of a single light beam, and a tracking error signal is obtained by detecting a variation in light distribution due to the interference of light diffracted by pits constituting the information track. A laser light beam emitted from a laser light source 21 is made incident upon a disc 26 having the information tracks by means of a lens 22, a half mirror 23, a reflection mirror 24 and an objective lens 25. The light beam reflected by the disc 26 is made incident upon a light detector 27 by means of the objective lens 25, the mirror 24 and the half mirror 23.
FIG. 5 is a signal processing circuit of the detection system shown in FIG. 4. As illustrated in FIG. 5, the light detector 27 comprises four light receiving regions 27A to 27D divided in a direction in which an image of the information track extends, i.e. a track direction Y and in a direction X perpendicular to the track direction Y and these regions are arranged in a plane of an exit pupil or an image plane of an exit pupil of the objective lens 25. Output signals from the diagonal regions 27A, 27C and 27B, 27D are summed in adders 28A and 28B, respectively. Then an information signal is derived from an adder 28C which forms a sum of the outputs from four regions 27A to 27D. A difference between the outputs signals from the adders 28A and 28B is derived by a subtractor 29 and the difference thus detected is multiplied in a multiplier 31 with an output signal from a phase shifter 30 which receives the information signal. Then an output signal from the multiplier 31 is transmitted through a low pass filter 32 to obtain a tracking error signal.
In the detection system shown in FIGS. 4 and 5, only the information signal and the tracking error signal can be derived from the single light beam. The focussing error signal has to be detected by separate light detecting means which receives a divided light beam and thus, the whole construction of the system is liable to be complicated and the signal gains might be decreased.
From Japanese Patent Application Laid-open Publication No. 93,223/77, it is known to derive from a single light beam, the tracking error signal, focussing error signal and information signal with the aid of the same principle as that of the detection system illustrated in FIG. 4. However, in this known system a light detector having a triangular shape has to be positioned at an end of an exit pupil of an objective lens and thus, a light beam could not be utilized efficiently and only a low signal to noise ratio can be obtained.
The applicant has proposed a method and apparatus which can detect a focussing error of an objective lens with respect to an object onto which a light spot is to be focussed as a light spot. According to this method, the optical system can be made small in size, the light detector can be easily arranged without need of precise adjustment and the focussing error can be detected very precisely. Such a method and an apparatus are described in the applicant's Japanese Patent Application No. 79,943/79 filed on June 25, 1979, laid open on Jan. 24, 1981, corresponding to U.S. patent application Ser. No. 195,075 filed on Oct. 8, 1980, as a CIP of U.S. patent application Ser. No. 161,428 filed on June 20, 1980, now U.S. Pat. No. 4,390,781.
FIG. 6 is a schematic view illustrating an optical pick-up apparatus comprising the above mentioned focus detection apparatus proposed by the applicant. In this apparatus, a linearly polarized light beam emitted from a laser light source 41 is collimated into a parallel light beam by a collimator lens 42 and passed through a polarizing prism 43 and a quarter-wavelength plate 44. Then, the parallel light beam impinges upon an objective lens 45 and is focussed on an information track of a disc 46 as a small light spot. The light beam reflected by the disc 46 is optically modulated in accordance with information recorded in the track and is reflected by the polarizing prism 43. The light flux reflected by the polarization prism 43 impinges upon a detection prism 47 having a reflection surface 48 and the light flux reflected by this surface 48 is received by a light detector 49. The reflection surface 48 is so arranged with respect to the incident light that under an in-focussed condition it makes a given angle with respect to the incident light (parallel light flux) which angle is equal to a critical angle or slightly smaller or greater than the critical angle. Now, for the time being, it is assumed that the reflection surface 48 is set at the critical angle. In the in-focused condition, the whole light flux reflected by the polarizing prism 43 is totally reflected by the reflection surface 48. In practice, a small amount of light is transmitted into a direction n shown in FIG. 6 due to incompleteness of a surface condition of the reflection surface 48. However, such a small amount of transmitted light may be ignored. If the disc 46 deviates from the in-focussed condition in a direction a in FIG. 6 and a distance between the objective lens 45 and the disc 46 is shortened, the light reflected by the polarizing prism 43 is no longer the parallel beam, but changes into a diverging light beam including extreme light rays ai.sub.1 and ai.sub.2. On the contrary, if the disc 46 deviates in the opposite direction b, the parallel light beam is changed into a converging light beam including extreme light rays bi.sub.1 and bi.sub.2. As can be seen in FIG. 6, light rays from an incident optical axis OP.sub.i to the extreme light ray ai.sub.1 have incident angles smaller than the critical angle and thus, are transmitted through the reflection surface 48 at least partially as illustrated by at.sub.1 (the reflected light being shown by ar.sub.1). Contrary to this, light rays between the optical axis OP.sub.i and the extreme light ray ai.sub.2 have incident angles larger than the critical angle and thus are totally reflected by the surface 48 as shown by ar.sub.2. In case of deviation of the disc 46 in the direction b, the above relation becomes inversed, and light rays below a plane which includes the incident optical axis OP.sub.i and is perpendicular to the plane of the drawing of FIG. 6, i.e. a plane of incidence, are totally reflected by the reflection surface 48 as denoted by br.sub.1, and light rays above said plane are at least partially transmitted through the reflection surface 48 as depicted by bt.sub.2. As explained above, if the disc 46 deviates from the in-focussed position, the incident angles of the light rays impinging upon the reflection surface 48 vary in a continuous manner about the critical angle except for the center light ray passing along the optical axis OP.sub.i. Therefore, when the disc 46 deviates from the in-focussed position either in the direction a or b, the intensity of the light reflected by the reflection surface 48 varies abruptly near the critical angle in accordance with the above mentioned variation in the incident angles as illustrated in FIG. 7. In this case, senses of the variations of the light intensities on both sides of said plane perpendicular to the incident plane and including the incident optical axis OP.sub.i vary in a mutually opposite manner. On the contrary, in the in-focussed condition, the light flux impinging upon the detection prism 47 is totally reflected by the reflection surface 48 and thus, the uniform light flux impinges upon the light detector 49. The light detector 49 is so constructed that the lower and upper light fluxes with respect to said plane are separately received by separate regions 49A and 49B, respectively. That is to say, the light detector 49 is divided along a plane which is perpendicular to the incident plane and includes an optical axis OP.sub.r of reflected light.
FIG. 7 shows a graph representing a variation of an intensity of reflected light in accordance with an incident angle near the critical angle. Curves R.sub.p and R.sub.s indicate the light intensities for P and S polarized light rays, respectively, P-polarized light and S-polarized light being defined with respect to an incident plane which contains a normal to an optical surface and a propagating direction of the light, wheren P-polarized light refers to an electric vector of polarized light vibrating in the incident plane and S-polarized light refers to an electric vector vibrating perpendicular to the incident plane. The curves are obtained when the detection prism 47 is made of material having a refractive index of 1.50. It should be noted that an intensity of a non-polarized light ray is equal to an intermediate value of (R.sub.p +R.sub.2)/2.
In FIG. 6, if the disc 46 deviates in the direction a, the light rays of the lower half of the incident light flux have incident angles smaller than the critical angle. Therefore, at least a part of the lower half light flux is transmitted through the reflection surface 48 and the amount of light impinging upon the light receiving region 49A is decreased. The upper half of the incident light flux has the incident angles larger than the critical angle and thus, is totally reflected by the surface 48. Therefore, the amount of light impinging upon the light receiving region 49B is not changed. On the contrary, if the disc 46 deviates in the direction b, the amount of light impinging upon the region 49B is decreased, but the amount of light impinging upon the region 49A is not changed. In this manner, the output signals from the regions 49A and 49B vary in an opposite manner. Therefore, a focussing error signal can be obtained by detecting a difference in output signals from the regions 49A and 49B and an information signal can be obtained by combining these output signals forming a sum signal.
The reflection surface 48 may be set at an angle slightly smaller than the critical angle. In such a case when the disc 46 deviates in the direction a, the amount of light impinging upon the region 49B is first increased and then becomes constant and the amount of light impinging upon the region 49A is decreased abruptly. Whereas, if the disc 46 deviates in the direction b, the amount of light impinging upon the region 49A is first increased and then becomes constant, while the amount of light impinging upon the region 49B is decreased abruptly.
In this manner by detecting a difference in output signals from the light receiving regions 49A and 49B, it is possible to obtain the focussing error signal having an amplitude which is proportional to an amount of the deviation from the in-focussed condition and a polarity which represents a direction of the deviation with respect to the in-focussed condition. The focussing error signal thus obtained is used to effect a focussing control for driving the objective lens 45 in the direction of its optical axis. Further, it is possible to derive the information signal corresponding to the pit information recorded in the information track as the sum signal of the output signals from the regions 49A and 49B. Further, in the in-focussed condition, since the light is scarcely transmitted through the reflection surface 48, a loss of light is very small and in the defocussed condition half of light flux with respect to the central light ray is totally reflected, but when the other half of light flux reflected by the surface 48 is decreased to a great extent, the difference in the amount of light impinging upon the regions 49A and 49B becomes great. Therefore, a very accurate focus detection can be effected with a very high sensitivity.
For instance, when use is made of an objective lens 45 having a numerical aperture NA=0.5 and a focal length f=3 mm and of a detection prism 47 having a refractive index n=1.50 and the disc 46 deviates by about 1 .mu.m, a variation of an incident angle for the extreme right ray which is subjected to the largest variation in incident angle about 0.015.degree. which causes a sufficiently large variation in light amount impinging upon the detector regions 49A and 49B.
In case of reading the information from the information record medium, such as a video disc, it is necessary to effect the focussing control as described above so that the light spot can be always focussed on the information record surface. It is also necessary to effect the tracking control so that the light spot can always follow a predetermined information track without deviating therefrom. In order to effect the tracking control, a tracking error signal has to be detected. However, in the above mentioned known system it is necessary to provide separate means for deriving the tracking error signal from the light reflected from the disc. Therefore, the optical system might become more complicated and the number of optical elements might be increased.